Comprehensive myeloid-derived suppressor cell (MDSC) immunophenotyping and functional assessment in chimeric antigen receptor (CAR) T therapy.
Authors: Xiaobei Wang, Parker Mathews, Krista Rowe Nichols, Shaima Jabbar, Johanna Jensen, Jonathan Huggis, Daniel Schrum, Erin Eberwein, Erin Kennedy, Christopher R. Kelsey, Taewoong Choi, Matthew McKinney, Ahmed Galal, Yubin Kang
Journal: Journal of Clinical Oncology (2023)

e19502 Background: CAR T cell therapy can provide sustained remissions in patients with hematologic malignancies. However, some patients are resistant to or relapse after CAR T therapy. CAR T therapy is complicated by cytokine release syndrome (CRS) and immune effector cell-associated syndrome (ICANS). There is an unmet need to understand immune cell types and functions in correlation with clinical outcomes. Methods: Patients with hematologic malignancies treated with commercial CAR T products were enrolled in the study. Blood samples were collected at day -7 before lymphodepletion, day 0 before CAR T infusion, day +7 after CAR T infusion and then every 3-6 months thereafter. Multi-color flow cytometry was performed. Total myeloid-derived suppressor cells (MDSCs, CD45 + CD3 − CD19 − CD20 − CD56 − CD16 − HLA-DR − CD33 + CD11b + cells), granulocytic (PMN)-MDSCs (CD14 - CD11b + CD15 + ), monocytic (M)-MDSCs (CD11b + CD14 + HLA-DR low/- CD15 - ), early stage (e)-MDSCs (Lin - HLA-DR - CD11b + CD33 + ), CD3 + , CD4 + , CD8 + T cells, B cells, and NK cells were quantitated. T cell proliferation in response to CD3 and CD28 stimulation was measured by Ki67 labeling. The function of MDSCs in suppressing T cell proliferation was measured by co-culturing isolated MDSCs with T cells in the presence of anti-CD3/CD28 beads for 4 days. CRS, ICANS and treatment response at 3 months were determined and correlated with immune cell types and functional assays. Results: At the time of this abstract submission, we have enrolled 20 patients in the study: 11 patients received axicabtagene ciloleucel, 1 tisagenlecleucel, 4 brexucabtagene autoleucel, and 3 ciltacabtagene autoleucel. The median age at CAR-T infusion was 69 years old; 16 were male and 4 female. The number of CD4 and CD8 T cells recovered in 1 week after CAR T infusion and the number of NK cells increased at 3 months after CAR T therapy. Interestingly, MDSC numbers remained low at 3 months after CAR T infusion. We also measured the function of MDSCs in suppressing T cell proliferation. Before lymphodepletion, MDSCs were very suppressive. Following CAR-T, MDSCs become less suppressive, allowing CD8 T cell proliferation. We found that a lower level of PMN-MDSCs at day 0 and +7 correlated with the development of CRS (p = 0.003, p = 0.0005). Lower PMN-MDSCs and less suppressive MDSCs at day +7 were associated with the development of ICANS (p = 0.009, p &lt; 0.0001). Higher percentage and proliferation of CD8 T cells and lower M-MDSCs at day +7 correlated with treatment response to CAR T therapy at 3 months (p = 0.02, p = 0.04, p = 0.0006). Conclusions: Our study shows the kinetics of immune cells and the function of MDSCs after CAR T therapy. Our preliminary data demonstrated a critical role of MDSCs in CAR T response and toxicities. Targeting MDSCs could be a novel approach for improving response in CAR T therapy.